Recently, public safety agencies have increasingly adopted a practice of equipping an officer with a mobile radio and a portable radio both operating on a same channel. While in the vehicle, the officer typically uses the mobile radio and while away from the vehicle, the officer uses the portable radio. In this manner, the officer is able to stay in communication whether the officer is in the vehicle or on foot away from the vehicle. This arrangement, however, may create an acoustic feedback problem in the vehicle when the officer transmits on one of the radios while the other radio is on. For example, when the officer is in the vehicle and transmits on the mobile radio while the portable radio is on, audio from the transmitting radio (mobile) is transmitted to a repeater and repeated to the portable radio. The audio may alternatively be transmitted directly from the mobile radio to the portable radio without the use of the repeater. The portable radio receives the audio and amplifies the audio through a speaker of the portable radio. When the audio is amplified through the speaker, feedback is created to a microphone of the mobile radio. This feedback is an acoustic feedback loop and creates a loud disruptive noise. Further, when feedback occurs, it disrupts the transmission from the transmitting radio thereby prohibiting the transmission from reaching an intended radio.
One technique used to resolve the acoustic feedback problem is to turn down the volume on the portable radio when transmitting on the mobile radio. While operational, this approach has several shortfalls. Turning down the volume of the portable radio may solve the noise problem, however, it may create a more significant problem. When the officer leaves the vehicle, the officer may easily forget to turn up the volume on the portable radio. By not turning up the volume on the portable radio, the officer is isolated from the communication system. The officer will not be able to hear messages being transmitted.
A second technique used to resolve the acoustic feedback problem involves a communication system where a user presses a button to initiate a muting operation. This action causes the mobile radio to send a mute request command to an infrastructure, typically, via a controlled channel. This mute request command includes an ID of a portable radio which is to be muted. The infrastructure in turn transmits an outbound muting command targeted for the portable radio. When muting is no longer desired, an unmute command is transmitted from the mobile radio to the infrastructure to reverse the process. This technique further includes a door switch system that causes the mobile radio to transmit the unmute command whenever the vehicle door is opened.
While this technique seems to work well, there are a few disadvantages. The technique relies on toggling commands which are repeated through an intelligent infrastructure. A one time message to mute or unmute the portable radio may be missed due to interference. The portable radio does not mute or unmute when desired if the portable radio does not receive the mute or unmute commands. Multiple retry operations may be specified in case a command does not get through on a first attempt. Retry operations cause the infrastructure to be loaded down with mute and unmute commands.
Further, once muted, the portable radio mutes all incoming messages indiscriminately. Depending on the implementation, a signaling failure may result in the portable radio not unmuting (if optional portable radio acknowledgment is not used) or the portable radio may create feedback for a time period during which the mobile radio is waiting and retrying to get a mute command through to the portable radio. Thus, communications to the portable radio may be eliminated by false muting or disrupted by a delay in execution of the mute function. Since the portable radio mutes all inbound transmissions, regardless of whether the transmission would create feedback or not, this significantly increases the possibility that a portable radio user may miss a message.
Relying on a door switch (or other change of state indicator) to disable the mute function is not a fool proof option since the officer may leave the vehicle without the automatic unmute function being executed. Officers often return to their vehicles, leave the door open, and use the mobile radio. If the officer presses a mobile mute button, uses the mobile radio, and then leaves the vehicle without closing and then opening the door, the portable radio remains muted. The officer may not be aware that the portable radio is still muted. When the officer leaves the vehicle, messages received by the portable radio are muted thus, the officer does not hear the messages. This could create a potentially dangerous situation for the officer if someone is trying to warn the officer of a life threatening situation. Furthermore, wiring a door switch additionally complicates installation of the mobile radio into the vehicle. The operation of pressing the mute button each time the officer returns to the vehicle (or opens the door) complicates the operation of the radio. The probability that the officer will forget to press the mute button may be high, especially when the officer is in a hurry or under stress.
Therefore, there is a need for an improved acoustic feedback avoidance method that is simple, that does not add additional loading to the infrastructure, and that does not rely on a user to press a button to activate.